Photoplethysmography (PPG) is based upon shining light into the human body and measuring how the scattered light intensity changes with each pulse of blood flow. The scattered light intensity will change in time with respect to changes in blood flow or blood opacity associated with heart beats, breaths, blood oxygen level (SpO2), and the like. Such a sensing methodology may require the magnitude of light energy reaching the volume of flesh being interrogated to be steady and consistent so that small changes in the quantity of scattered photons can be attributed to varying blood flow. If the incidental and scattered photon count magnitude changes due to light coupling variation between the source or detector and the skin or other body tissue, then the signal of interest can be difficult to ascertain due to large photon count variability caused by motion artifacts. Changes in the surface area (and volume) of skin or other body tissue being impacted with photons, or varying skin surface curvature reflecting significant portions of the photons may also significantly impact optical coupling efficiency. Physical activity, such a walking, cycling, running, etc., may cause motion artifacts in the optical scatter signal from the body, and time-varying changes in photon intensity due to motion artifacts may swamp-out time-varying changes in photon intensity due to blood flow changes. Each of these changes in optical coupling can dramatically reduce the signal-to-noise ratio (S/N) of biometric PPG information to total time-varying photonic interrogation count. This can result in a much lower accuracy in metrics derived from PPG data, such as heart rate and breathing rate.
An earphone, such as a headset, earbud, etc., may be a good choice for incorporation of a photoplethysmography (PPG) device because it is a form factor that individuals are familiar with, it is a device that is commonly worn for long periods of time, and it is often used during exercise which is a time when individuals may benefit from having accurate heart rate data (or other physiological data). Unfortunately, incorporation of a photoplethysmography device into an earphone poses several challenges. For example, earphones may be uncomfortable to wear for long periods of time, particularly if they deform the ear surface. Moreover, human ear anatomy may vary significantly from person to person, so finding an earbud form that will fit comfortably in many ears may be difficult. Other form-factors for PPG devices, such as wristbands, armbands, clothing, and the like may be problematic for motion artifacts as well.
Some previous efforts to reduce motion artifacts from wearable PPG devices have focused on passive filtering, adaptive filtering (U.S. Pat. No. 8,923,941), or removing noise associated with footsteps (U.S. Pat. No. 8,157,730) with harmonics associated with a subject's cadence (US 2015/0366509). Each of these methods is associated with both strengths and weaknesses, depending on the wearable PPG sensor location or the type of motion noise that is present for the subject wearing the PPG sensor.